What are the neurodevelopmental outcomes of children with asymptomatic congenital cytomegalovirus infection at birth? A systematic literature review

Abstract Congenital cytomegalovirus (cCMV) is among the most common congenital infections globally. Of 85%–90% cCMV‐infected infants without symptoms at birth, 10%–15% develop sequelae, most commonly sensorineural hearing loss (SNHL); their childhood neurodevelopmental outcomes are less well understood. Embase and MEDLINE were searched for publications from 16th September 2016 to 9th February 2024 to identify studies reporting primary data on neurodevelopmental outcomes in children with asymptomatic cCMV (AcCMV), measured using assessment tools or as evaluated by the study investigators, clinicians, educators, or parents. The Newcastle‐Ottawa scale was applied to studies to assess risk of bias. Of 28 studies from 18 mostly high‐income countries, there were 5‐109 children with AcCMV per study and 6/28 had a mean or median age at last follow‐up of ≥5 years. Children with AcCMV had better neurodevelopmental outcomes than children with symptomatic cCMV in 16/19 studies. Of 9/28 studies comparing AcCMV with CMV‐uninfected children, six reported similar outcomes whilst three reported differences limited to measures of full‐scale intelligence and receptive vocabulary among children with AcCMV and SNHL, or more generally in motor impairment. Common limitations of studies for our question were a lack of cCMV‐uninfected controls, heterogeneous definitions of AcCMV, lack of focus on neurodevelopment, selection bias and inadequate follow‐up. There was little evidence of children with AcCMV having worse neurodevelopmental outcomes than CMV‐uninfected children, but this conclusion is limited by study characteristics and quality; findings highlight the need for well‐designed and standardised approaches to investigate long‐term sequelae.


| INTRODUCTION
Prevalence at birth of congenitally acquired CMV (cCMV) is estimated at 0.48% (95% CI 0.40%-0.59%) in high-income countries and 1.42% (0.97%-2.08%) in low and middle-income countries, where maternal seroprevalence is higher, making this among the most frequent congenital infections globally. 1,2At birth, around 10%-15% of cCMV-infected newborns have clinically detectable symptoms of cCMV, of whom an estimated 40%-58% experience persistent sequelae including sensorineural hearing loss (SNHL), ophthalmological outcomes, and neurodevelopmental delay. 35][6] Children with asymptomatic cCMV (AcCMV) at birth have a lower risk of long-term sequelae (estimated to occur in 10%-15% 3 ) but account for the majority of children with cCMV-related sequelae of any severity overall, owing to this group's much larger size (Figure 1).Most infants without symptoms at birth are likely to remain undiagnosed in the absence of universal screening.SNHL is the most commonly reported sequela attributed to cCMV infection, 7 with permanent childhood hearing impairment reported in at least 40% of children with symptomatic cCMV (ScCMV) at birth, and up to 30% in children with AcCMV at follow-up between 6 months and 18 years, in a recent review. 8r children with cCMV but without apparent symptoms at birth (i.e.85%-90%), the implications of cCMV for outcomes other than hearing, such as neurodevelopmental outcomes, are less clear.Early studies in which children with cCMV were systematically identified through screening and prospectively followed up to at least 3 years of age showed that for those without symptoms of cCMV, neurodevelopment was similar to uninfected controls. 9,10A systematic review conducted in 2016 by Bartlett et al. 11 found that of 11 studies published between 1974 and 2016 comparing neurodevelopmental outcomes in children who had AcCMV at birth versus uninfected control groups, eight studies reported no difference at follow-up age between 10 months and 12.5 years.Of the remaining three studies, differences were based on outcomes of a single child, did not persist over time or were among children in whom hearing loss had not been excluded as a potential confounder of the association between AcCMV and poorer verbal Intelligence Quotient (IQ). 11However, only five of these 11 studies had follow-up to at least four years of age (439 controls and 397 AcCMV total across the five studies), and methodological heterogeneity limited comparisons between (and possibly within) studies and generalisability of findings to a contemporary population.With the introduction of newborn hearing screening, children with hearing loss in the absence of other apparent symptoms in the newborn period are now increasingly included in the 'symptomatic at birth' group, although this categorisation is applied inconsistently, 11 which also poses challenges in comparing other, related neurodevelopmental outcomes (e.g.speech and language development) across groups and studies.
A complete picture of the implications of cCMV for the longer term health and development of children without symptoms at birth is needed to inform future research and policy, including around screening of newborns for cCMV and potential treatment and vaccination strategies.We systematically reviewed literature published since the last review by Bartlett et al. 11 on neurodevelopmental outcomes of children with AcCMV.

| METHODS
This review was conducted in line with the PRISMA, 2020 statement 12 and registered in the International Prospective Register of Systematic Reviews (PROSPERO; registration number: CRD42022331535 13 ).Our methodology is adapted from an earlier F I G U R E 1 Estimated number of children with long-term sequelae associated with congenital CMV infection, by presence of clinically detectable symptoms at birth, per 100,000 births in high income and low or middle-income countries.Estimates of cCMV prevalence from 1 and estimates of proportions with symptoms at birth and long-term sequelae from. 3Numbers rounded to nearest 5.
review by Bartlett et al. 11 where acknowledged.Embase and MED-LINE databases were searched for articles published from 16   September 2016 to 6 May 2022, subsequently updated to 9 February 2024, to capture papers published since the earlier review.The full search strategy can be found at: https://www.crd.york.ac.uk/PROS-PEROFILES/331535_STRATEGY_20240313.pdf. 13o reviewers independently screened article titles and abstracts and, for articles not excluded, the full text.Data extraction was independently validated by two reviewers, with conflicts discussed and resolved by a third reviewer.

| Definitions
cCMV infection was defined as a positive CMV result diagnosed through viral isolation by culture and/or identification of viral DNA through polymerase chain reaction in neonatal saliva and/or urine and/or blood sample taken within the first 21 days of life, while CMVuninfected was defined as a negative result on this same category of sample. 14 considered as 'asymptomatic' (hereafter referred to as 'AcCMV'), a child with a positive CMV result but without clinically detectable symptoms of CMV at birth, according to the definition used in each included study.Unlike the earlier review, 11 we did not exclude studies that categorised children with abnormalities detected through further investigations (e.g.neuroimaging) as 'asymptomatic', because we are interested in the outcomes of all children who, in the absence of newborn CMV screening, may not be diagnosed.
Neurodevelopmental impairment was taken as defined by the individual studies, so included any below average performance on developmental, neurological, or cognitive assessment tools (or using stricter cut-offs), or as evaluated by the study investigators, clinicians, educators, or parents.

| Eligibility criteria
Studies included were quantitative observational studies with followup data beyond 30 days of life on neurodevelopment outcomes (neurological, speech and language, cognitive or motor) among children with AcCMV at birth.We did not include studies of children with neurodevelopmental delay who had been retrospectively tested for cCMV unless a comparison group without the outcome had also been included (i.e. to allow the neurodevelopmental delay attributable to cCMV to be estimated).Case reports and case series were excluded, as were studies with outcomes reported for <3 children with AcCMV, non-primary studies and those not published in English.
Conference abstracts were included only when linked to a full-text article.For multiple publications from the same cohort, only the one with the largest population was included, unless different papers contained information on different outcomes, in which case this is indicated in the results.Studies reporting solely on other outcomes (e.g.hearing, vestibular dysfunction) or not disaggregating outcomes for children with AcCMV were excluded.Our eligibility criteria differed from Bartlett et al 11 in places: we included studies in which some children categorised as AcCMV had received antivirals (rather than making this part of our exclusion criteria) and extracted data on antiviral use; we placed a minimum restriction on follow-up duration (30 days or more); we did not exclude retrospective cohort studies of children with neurodevelopmental delay, provided other criteria were met, and we used a broader definition of AcCMV (see definitions section).
Information extracted from studies included study design, characteristics and population, reason for CMV testing (e.g., screening and/or diagnostic investigation), the CMV diagnostic tests used, antiviral treatment received, method(s) for assessing neurodevelopmental outcomes, definitions used for ScCMV (and therefore AcCMV), and for neurodevelopmental impairment, duration of follow-up/age at assessments, and results for neurodevelopmental outcomes.Neurodevelopmental outcomes were stratified by hearing status where available.

| Assessing neurodevelopment and follow-up
Early child development is defined by the WHO as the period from conception to 8 years, 15 marked by gross motor abilities, fine-motor coordination, and development of language abilities. 16Assessments to identify neurodevelopmental delays have limitations at the youngest ages (floor effects) and problems may be more likely to be identified at older ages, including school entry. 16Neurodevelopmental outcomes can be measured using a variety of methods including validated assessment tools (sometimes requiring administration by a professional), or evaluations by clinicians, parents or educators.Outcome measures are related to the type of study and population-for example, serial measures using assessment tools are likely to only be available in prospective studies or in children with a clinical indication for follow-up.
A post-hoc decision was made to conduct statistical tests (Fisher's exact or chi-squared tests) to compare outcomes in AcCMV and cCMV-uninfected groups, where an included publication reported sufficient data but with no statistical test.

| Risk of bias assessment
The Newcastle-Ottawa Scale (NOS) for assessing the quality of nonrandomised studies was used to assess risk of bias in the studies, including selection bias in the study population and in measurement of exposure and outcome, bias in the availability of follow-up information, adequacy of follow-up duration and comparability of the AcCMV and comparison groups where applicable. 17For the purposes of this review, follow-up to at least age 5 years was scored as 'adequate' and defined as a mean or median follow-up period of at least 5 years or assessments scheduled to at least 5 years within each study.SMYRLI ET AL.
-3 of 23 Of 1436 articles published between September 2016 and February 2024, 1408 remained after de-duplication and 81 articles underwent full-text assessment; after agreement between two reviewers, 28 met inclusion criteria (Figure 2).

| Study characteristics
Table 1 shows the characteristics of 28 studies included in this review.
T A B L E 1 Study characteristics of the 28 studies included in the systematic review.

| Neurodevelopmental outcomes in studies with 5 years or more of follow-up
Six studies (two from the same cohort) reported outcomes with a mean or median follow-up or age at scheduled assessment of at least 5 years, [40][41][42][43][44][45] highlighted in red in T A B L E 2 Outcome assessment, length of follow-up and neurodevelopmental outcomes of 28 studies included.showed a higher prevalence of cognitive, motor and speech-language impairment in the cCMV infected group (no difference for neurological impairment) (Table 2).On comparing the proportion with each impairment in the AcCMV and CMV-uninfected groups using data from the paper, differences in neurological, cognitive or speech language impairment were not statistically significant, but children with AcCMV were more likely to have motor impairment (10.3% vs. 1.5% among cCMV-uninfected, p < 0.01, no data on vestibular function). 43rent-reported measures showed a similar proportion of children with each delay between the cCMV-infected group (AcCMV and ScCMV combined) and the cCMV-uninfected, with the exception of expressive language (delays in 26.1% of ScCMV, 11% of AcCMV and 5.8% of uninfected) and general development (delays in 21.7% of ScCMV, 8% of AcCMV, 3.7% of uninfected).When we conducted statistical tests to compare the proportion with each delay in the AcCMV versus cCMV-uninfected groups, the only one with weak evidence of an association was expressive language (p = 0.092) (all others, p > 0.1). 44

| Risk of bias assessment
Seven studies had no comparison group, therefore reducing the denominator for the scale from 7 to 5, 18,19,32,36,37,39,45 as seen marked with ' a ' in Table 2; of the remainder, most compared children with [40][41][42][43][44] Frequent sources of bias were that selection of study populations was not representative (e.g., neonates were tested for cCMV due to symptoms), short follow-up periods, and that study groups were difficult to compare due to a lack of information on other factors, for example, presence and degree of hearing loss.

| DISCUSSION
In this review of 28 studies published in 2016-2022 reporting on neurodevelopmental outcomes of children with AcCMV, only nine compared outcomes between AcCMV and uninfected children.Six of these nine studies reported similar outcomes 24,[33][34][35]40,42 (the two with follow-up of at least 5 years were from overlapping cohorts), while three did report some differences, either limited to measures of full-scale intelligence and receptive vocabulary among children with AcCMV and SNHL, 41 or more generally in motor impairment and with borderline association for expressive language. 43,44These latter two studies, published from the same retrospective cohort of sixyear-olds in the Netherlands, did not report these statistical comparisons but reported data that allowed us to conduct these tests.Of note, overlap between hearing and motor impairment was not reported, but only three children with AcCMV had hearing loss while 11 had motor impairment; prevalence of hearing loss in these studies (collected from routine medical data) was lower than expected, and undiagnosed hearing loss or vestibular dysfunction could be linked with the motor and language delays observed among children with AcCMV.
Comparisons of neurodevelopmental outcomes between AcCMV and ScCMV were more widely reported (by 19 studies) and showed, as in previous literature, 46,47 that adverse outcomes were generally less common among children with AcCMV.The magnitude of this difference varied widely-likely in part driven by selection criteria for the included populations.Seven studies reported outcomes in children with AcCMV without any comparison group and were mostly small; five reported no adverse developmental outcomes, 18,36,37,39,45 while two reported developmental delays in 1/79 and 8/54 children respectively with AcCMV. 19,32e earlier review by Bartlett et  impaired language development) in AcCMV compared to controls. 49me of the main study limitations, such as short follow-up times and lack of comparison groups, were similar across the reviews.In our review, although the retrospective cohort in the Netherlands had one of the stronger designs, 43  or timing of maternal CMV infection, 21,23,24,26,28,[30][31][32][33][34][35]45 reflecting the challenges in collecting this data and the design of many studies which were either retrospective or followed infants from birth.
Almost all studies included in this review were conducted in highincome countries, where a greater proportion of pregnant women than in low or middle-income countries are at risk of a primary infection (averaging 30% in the European region). 51In a large population-based prospective cohort study in Finland included in our review, 48% of children with cCMV were born to women with a primary CMV infection, 34 similar to 52% from a previous French study, 52 while this proportion was 4/15 and 1/10 in prospective cohorts in Spain and Greece respectively, 21,45 and none of the 123 children in a South Africa cohort with known maternal infection type. 33Although vertical transmission risk is much higher following maternal primary infection, the risk of adverse outcomes among infants with cCMV seems similar regardless of maternal infection type. 53In our review, four studies of children born to women with primary CMV infection showed generally quite small numbers of AcCMV with neurodevelopmental impairment at follow-up (none, 35 2/19, 23 1/18, 24 8/54 32 ) while a larger study stratifying outcomes by maternal infection type showed that all 6/70 AcCMV with neurodevelopmental impairment were born to mothers with primary maternal CMV infection. 26Although first trimester infection is important for risk of symptoms at birth, 54 less is known about maternal timing of infection and later outcomes among children with AcCMV, and whether maternal serology is relevant to outcomes in this group.

| Limitations
The interpretation of the included studies was limited by mostly short and highly variable follow-up times, non-representative samples of differing sizes (arising from different study designs-for example, populations screened for cCMV vs. those with clinical indications for testing), inconsistent definitions of AcCMV, and lack of an uninfected control group in many of the studies.Follow-up times of less than 5 years, as in the majority of studies, will result in underascertainment of relevant outcomes identified during school years and among groups more likely to experience diagnostic delays (e.g.girls with ASD 55 ), and reduce statistical power to detect differences between groups.On the other hand, in prospective studies, children already diagnosed with cCMV may be more intensively monitored than cCMV-uninfected controls, while loss to follow-up can be more common among children with AcCMV than ScCMV 27 and possibly associated with probability of later outcomes, introducing selection bias into study findings.The lack of cCMV-uninfected comparison groups in the majority of studies limits conclusions about the extent to which adverse outcomes are attributed to cCMV, especially given that some of the outcomes explored are relatively common (e.g.ASD diagnosed in 1.76% of children in the general population in England overall by mean age 10.18 years 56 ).In this review, the synthesis of results was also limited by the large variation in how neurodevelopmental impairment was defined and evaluated.Vestibular dysfunction can occur in children with cCMV both with and without hearing loss, and may contribute to motor delay 56 ; however it is likely to be underdiagnosed, possibly particularly in children with AcCMV without hearing loss, and data on co-occurring vestibular dysfunction was very limited in the studies included in this review. 57We only included studies published in English in this review, which is a limitation of our inclusion criteria.We also excluded grey literature, given that this is more likely to be low quality and difficult to interpret.

| Implications for practice and policy-Explicit recommendations for future research
This review highlights the continued need for more robust, longerterm data to understand neurodevelopment of children with AcCMV.In the UK, the annual cost of managing individuals of all ages with cCMV has been estimated at £732 million 58 but this is based on an incomplete understanding of long-term effects in infants without symptoms at birth, who may never be diagnosed despite experiencing sequelae.A better understanding of the longterm neurodevelopmental outcomes of AcCMV has wide-ranging implications including for informing screening policy, the costeffectiveness of future vaccine implementation (with a phase 3 CMV vaccine trial by Moderna currently underway 59 ), and inclusion of cCMV testing into protocols for investigation of neurodevelopmental delay/disorder, which requires availability of routine DBS samples from the newborn period.Inconsistencies in outcome measures and lack of statistical power can be addressed through shared protocols and multi-centre studies, as already shown in the Spanish studies. 20,21The Spanish database is now the basis for the European Registry, which already has more than 1050 children In conclusion, there remains little current evidence indicating that children with AcCMV are at higher risk of experiencing neurodevelopmental sequelae compared to cCMV-uninfected controls from the 28 studies included in this review.However, this conclusion is limited by differing neurodevelopment assessment methods, short follow-up times, a lack of comparison groups and possible bias in study inclusion and/or outcome ascertainment.

Figure 2 .
Papaevangelou et al. reported normal psychomotor development, receptive and expressive language skills on the Griffiths mental development scale among five children with AcCMV and was the only study of the six not to include a CMV-uninfected comparison group.45Three papers reported on different outcomes from overlapping cohorts arising from a CMV screening study in the USA of children born in 1982-1992.[40][41][42]No reports of IQ scores below 70 were F I G U R E 3 Length of follow-up of study participants presented as mean, median or scheduled age at last assessment (in months) for each study included in the systematic review.x = Age at which results were obtained (predetermined assessment point or through active monitoring), bars indicate range of ages at last assessment, † = Mean age at last assessment, ‡ = Median age at last assessment, n = number of AcCMV with follow-up, * = follow-up length reported for AcCMV group only (whereas others are length of follow-up for all study participants with follow-up), Red 'x' or ranges: Studies with follow-up of mean/median or scheduled assessment of 5 or more years.a Bartlett et al.: Unclear follow-up duration but minimum 30 days after birth for AcCMV group (number of AcCMV with FU = 79).b Jin et al.: Longitudinal study was up to 18 years but no clear indication of age at neurodevelopmental assessment (arrow indicates that duration of follow-up exceeds x axis).c Korndewal et al.: Data retrospectively collected and represent cumulative total of impairments diagnosed up to the first 6 years of life (rather than diagnosed/present at 6 years of age).d Lopez et al.: Median age at last assessment was 17 years for all tests and groups, apart from expressive vocabulary which was 13 years (arrow indicates that duration of follow-up exceeds x axis).e Papaevangelou et al.: 8 children assessed at 12 months of age and 5 of these were assessed at 5 years of age (12 months and 5 years follow-up plotted as a 'x').f Topham et al.: Outcome was assessed at minimum one point between the ages of 18 years of age (indicated on plot with arrow from 72 months onwards).

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SMYRLI ET AL.included.60However, observational studies of diagnosed children will continue to focus on infants with hearing loss at birth and the ScCMV group.Universal neonatal screening for cCMV has been recently introduced in Minnesota, Ontario and Saskatchewan,61 creating an opportunity for collection of outcome data irrespective of maternal diagnosis or neonatal symptoms.Study designs are needed that focus on clearly defined neonatal and longer-term outcome measures of children with AcCMV, including degrees and timing of hearing loss, rehabilitation, and vestibular function measures alongside neurodevelopment.A better understanding of prognostic neonatal biomarkers (whether brain imaging, transcriptomic patterns, or others), as well as robust neurodevelopmental measures are required to fully understand the true spectrum of the longer-term effects of AcCMV.

First author (year of publication) Country Study design Study period Study population Study population with follow-up data
T A B L E 1 (Continued)First author (

year of publication) Country Study design Study period Study population Study population with follow-up data
-82 fetuses: 24 TOP and 58 cCMV-positive neonates (81/82 born to mothers with a primary and 1/82 to a nonprimary CMV infection) -58 cCMV-positive cases at birth: 47 AcCMV (38 with no US features; 9 with non-severe features) and 11 ScCMV (1 had severe and 10 non-severe US findings in utero) -46/47 AcCMV and all 11 ScCMV had FU 29 Lin (2020) First author (

year of publication) Country Study design Study period Study population Study population with follow-up data
T A B L E 1 (Continued)First author (

First author (year of publication) Country Study design Study period Study population Study population with follow-up data
while the other reported mildly abnormal neurological evaluation in 6/54 AcCMV (but none with more severe abnormalities), and 14% (8/54) with motor T A B L E 1 (Continued) Abbreviations: AcCMV, children with asymptomatic congenital cytomegalovirus; cCMV, congenital cytomegalovirus; FU, follow-up; ND, neurodevelopment (al); ScCMV, children with symptomatic congenital cytomegalovirus; SNHL, sensorineural hearing loss; TOPs, Termination of pregnancy; US, ultrasound.a Studies with overlapping populations, therefore combined in Table 1.The study populations are disaggregated in cells accordingly.

of bias scoring (NOS) Included in author's definition of symptomatic cCMV? Hearing loss † Brain imaging abnormalities †
42 risk of bias scoring = no comparison group available, therefore denominator was 5 instead of 7, with a score of 7/7 meaning minimal bias in the Newcastle-Ottawa Scale tool.reportedamong76children with AcCMV and 29 cCMV-uninfected children by Topham et al. and there was no significant difference in attention problems or hyperactivity t-scores ≥65 between the two groups.42Inan overlapping cohort, Lopez et al. compared combined IQ scores at later ages (up to 18 years) according to symptoms of cCMV at birth and hearing at age 2 years.They reported no dif- a